1. Field of the Invention
This invention relates generally to rotary engines, and more particularly, to a rotary engine having vanes which are radially slidable through a rotor.
2. Description of the Related Art
The vast majority of automotive power plants and utility engines employ reciprocating four or two stroke spark ignition (SI) or compression ignition (CI) engines. Both two and four stroke reciprocating engines suffer from a low working gas displacement to physical engine displacement (size and weight) ratio, high inertia loads, and require complex mechanical arrangements. These complex arrangements include reciprocating pistons, crankshafts, cam shafts, and high speed valve trains. New two stroke reciprocating automotive engine designs are being explored as potential means to increase the power/weight and power/volume densities by as much as a factor of two. Even this two stroke approach, having simultaneous intake and exhaust processes, presents a severe challenge in producing a clean-burning engine.
The Wankel engine is the only mass production rotary engine. This engine provides a slightly better (by up to a factor of two) horsepower/volume and horsepower/weight ratio than reciprocating engines. However, the Wankel engine is similar to other prior art two or four stroke engines in terms of design complexity and suffers from relatively poor fuel efficiency due primarily to poor fuel mixing and combustion. These characteristics result in the Wankel being preferable to conventional reciprocating engines in only very limited applications.
Recently, axial vane rotary engines have been explored. Such engines employ a plurality of axially slidable vanes on a rotor which are circumferentially spaced apart. The vanes reciprocate back and forth as the rotor rotates, by cooperation with a cam surface disposed on each side of the rotor. When the vanes slide towards the rotor on one side thereof, the space between the rotor and the motor housing decreases, thus compressing the gas. Examples of axial vane rotary engines are disclosed in U.S. Pat. No. 4,401,070 to McCann, and U.S. Pat. No. 3,819,309 to Jacobs.
Axial vane rotary engines yield an increase in power/weight and power/volume ratios over either Wankel or reciprocating engines due to their high ratio of internal displacement volume to external volume and reduced weight. In addition, axial vane rotary engines are expected to yield smoother operation than prior art designs due to their many (typically twelve) chambers and power strokes on each revolution. Such engines should also result in reduced nitrous oxide emissions due to their ability to rapidly quench combustion gases. These improvements in conjunction with their inherently simplified manufacturing represent a substantial improvement over prior art engines.
Notwithstanding these expected benefits, axial vane rotary engines have several drawbacks. The axially displaced vanes experience large loads along multiple sliding surfaces. Centripetal forces produce high loads along the exterior periphery, while cam acceleration forces result in large forces along the upper and lower cam surfaces. These load paths lead to larger friction and sealing problems than if the loads all originated from a single direction. In addition, since these forces increase in magnitude in both directions with engine scale, it is difficult to scale the engine. The centripetal loads limit the radius and speed of the engine, whereas the axial acceleration forces limit the axial dimensions of the engine. As a result, multiple stacks of rotor modules are typically required to provide large displacement engines. The load problem is partially offset by stacking rotor modules, but this yields a more complex design. Another problem with axial vane rotary designs is similar to that experienced with the Wankel design, that of engine thermal fatigue due to localized hot spots where combustion occurs.
While the high horsepower/weight and horsepower/volume characteristics of the axial rotary engine will allow high output engines, such engines must be highly throttled for off-peak demand. This compromises fuel efficiency if conventional throttling techniques are employed or alternatively requires more complex transmissions to reduce engine speed during off-peak use and subsequent engine power output. The transmission approach, however, may lead to poor throttle response when required. Moreover, the axial vane engine does not provide a way to control engine displacement to tailor the engine's effective size to the required power demand.
These designs are further limited in efficiency since they provide only a topping cycle operation and do not provide a way to recover exhaust heat by utilizing a bottoming thermodynamic cycle in the same engine. As a result of these and other deficiencies, none of the above conventional engines (reciprocating, Wankel, or axial vane rotary) provide better fuel efficiency than only about 25% mechanical (shaft) power out/heat in for spark ignition and 35% for compression ignition engines.